Unreferenced English articles’ translation quality-oriented automatic evaluation technology using sparse autoencoder under the background of deep learning

Currently, both manual and automatic evaluation technology can evaluate the translation quality of unreferenced English articles, playing a particular role in detecting translation results. Still, their deficiency is the lack of a close or noticeable relationship between evaluation time and evaluation theory. Thereupon, to realize the automatic Translation Quality Assessment (TQA) of unreferenced English articles, this paper proposes an automatic TQA model based on Sparse AutoEncoder (SAE) under the background of Deep Learning (DL). Meanwhile, the DL-based information extraction method employs AutoEncoder (AE) in the bilingual words’ unsupervised learning stage to reconstruct the translation language vector features. Then, it imports the translation information of unreferenced English articles into Bilingual words and optimizes the extraction effect of language vector features. Meantime, the translation language vector feature is introduced into the automatic DL-based TQA. The experimental findings corroborate that when the number of sentences increases, the number of actual translation errors and the evaluation scores of the proposed model increase, but the Bilingual Evaluation Understudy (BLEU) score is not significantly affected. When the number of sentences increases from 1,000 to 6,000, the BLEU increases from 96 to 98, which shows that the proposed model has good performance. Finally, the proposed model can realize the high-precision TQA of unreferenced English articles.


Introduction
Human society has seen progress and development through cultural exchange and collision. As the main carrier of different cultural exchanges, the translation between different languages is the key to multicultural exchanges. Therefore, translation technology is indispensable in disseminating cultural information [1,2]. In the era of global economic integration, translation technology has become one of the industries attracting worldwide attention, which has promoted the development of the country's foreign economy. Machine Translation (MT) tasks began in the 1950s. Since the 1990s, MT has entered a period of rapid development thanks to soaring computing performance, storage capacity, and more available corpus. Meanwhile, with the extensive development of the MT system, MT evaluation has also risen sharply. It has become an active research field and a hot topic for discussion [3][4][5]. Scholars try to understand how well MT results are and whether they are eligible to replace manual translation. At the same time, by evaluating MT, researchers can intuitively demonstrate the MT system's performance for optimization and fine-tune. Since manual labor demands considerable time and money, the automatic Translation Quality Assessment (TQA) method came into being. Most commonly, calculating the similarity between the MT results and manual works can improve the MT system performance. Besides, the comparison can partially reflect the quality of MT works [6].
In the current MT-oriented Translation Quality Assessment (TQA), the automatic TQA technology for artificial translation of reference-based English articles is widely used. Nevertheless, the evaluation results depend entirely on the given reference articles, which focus on reflecting the performance of the Machine Translation Systems (MTSs). However, translation references are not necessarily correct and practical [7]. Some scholars have also applied fuzzy logic systems and Deep Learning (DL) technology to non-artificial translation reference-based (unreferenced) English articles translation. They find that although it can roughly express the meaning of the article, there are still some defects in the "faithfulness, expressiveness, and elegance" of the translated language. If these technologies are to be applied to translation, they need continuous improvement and optimization [8]. Therefore, more and more researchers have begun to study the new MT-oriented TQA technology for unreferenced English article translation. Accordingly, this paper compares the unreferenced English articles-oriented and referenced English articles-oriented TQA. It finds that the unreferenced English articles-oriented TQA poses no standard restrictions on reference articles and can be applied in the research stage and be popularized in practical applications [9,10]. Apparently, such an unreferenced English articles-oriented TQA is more practical and has more research value.
At present, the unreferenced English articles-oriented TQA technique is still in the early stage of development. It is not technologically mature, but there is room for growth and progress in practical applications. In summary, the current research shows that China has not paid enough attention to the TQA technology of unreferenced English articles compared with European and American countries. Accordingly, under the background of Deep Learning (DL), this paper proposes an automatic TQA model for unreferenced English articles based on Sparse AutoEncoder (SAE): the DL algorithm is applied to TQA and Feature Extraction (FE) to realize the high-precision TQA for MT. calculate the net input and output of the output layer and each hidden layer, and then calculate the prediction results of the neural network [14,15]. If there is a large error between the calculated prediction result and the expected output, the back-propagation of the error is started. In the process of error back-propagation, it is necessary to input the error back into the output layer, back-propagate the error, and update the weight according to the error of each layer. Simultaneously, the weight will be updated and adjusted continuously. Until the learning process reaches the preset number of iterations or the output error is less than the specified threshold, the learning process is terminated [16,17]. Fig 1 draws the structure of BPNN, including the input layer, hidden layer, and output layer. The hidden layers can be one or multiple, each with several nodes.
In the process of forwarding calculation, when the data in the input layer of BP NN is x 1 , x 2 , x 3 ,. . .,x n , the input of each neuron in the hidden layer is calculated by Eq (1): In Eq (1), n, m-the number of neurons in the input layer and hidden layer. w ij -connection weight between j neuron in the input layer and i neuron in the hidden layer. p i -the threshold of neurons in hidden layer i. h i -input of i neuron in the hidden layer.
The hidden layer adopts the Sigmoid function as the Activation Function (AF) of the neurons, so the output of neurons in the hidden layer can be expressed by Eq (2): In Eq (2), the AF f x ð Þ ¼ 1 1þe À x . The output layer adopts the identity function as the AF of neurons, and the threshold is 0. Then the output of each neuron in the output layer can be expressed by Eq (3): In Eq (3), v ki -connection weight between i neuron in the hidden layer and k neuron in the output layer. l-the number of neurons in the output layer.
The connection weights between the neurons in the input layer and the hidden layer form the weight vector W. After the weight vector W is determined, the output can be calculated according to the input of the neural network.
The learning samples of the back-propagation are (x 1r , x 2r ,. . .,x nr ; t kr ), r = 1,2,. . .,r, where r is the number of samples. After the weight vector W is given, the actual output y kr of the neural network can be calculated, and the error functions of output error and learning sample r are defined as d kr ¼ t kr À y kr ; e r ¼ 1 The value of weight vector W is random in the initial case, so the actual calculated output y kr of the network is not high. After the neuron number m is determined in the hidden layer, the error d kr can be reduced by adjusting the W. Back-propagation is along the function e r and adjusts the weight vector with the negative gradient direction [18][19][20]. Then, the correction value of weight vector W is set as ΔW and calculated as DW ¼ À Z @e r @W , where η is the learning rate (0-1). Eqs (4)-(8) are obtained through calculation: Dv ki ¼ Zd kr o ir ð6Þ Next, ΔW can be calculated by Eqs (4)- (8). Then, the weight vector is modified according to W = W+ΔW [21].
(2) Auto Encoder (AE) algorithm. AE is a typical three-layer neural network structure proposed by Rumelhart. The network structure includes an input layer, an output layer, and a hidden layer. The dimension of the output and the input layer is both n, and the dimension of the hidden layer is m [22,23]. Fig 2 gives the structure of an AE: The encoding process refers to the process from the input layer to the hidden layer, and the decoding process refers to the process from the hidden layer to the output layer. If the encoding function and decoding function are set to f and g, respectively, Eqs (9) and (10) can be obtained: In Eqs (9) and (10), s f -AF of the encoder, usually Sigmoid function, calculated by f x ð Þ ¼ 1 1þe À x . s g -AF of decoder, usually taking identity function or Sigmoid function. wweight matrix between the hidden layer and input layer.w-the weight matrix between the output layer and the hidden layer, andw ¼ w > : y ¼ fw; p; qg is the parameters of AE.
The output Y of the output layer can be regarded as the predicted value of the input X of the input layer. AE can use the BP algorithm to adjust the neural network parameter. When the error between the input X and the output Y is within the acceptable range, AE retains most of the information of the original input data. At this time, the training process of the AE neural network is completed [24]. The proximity between X and Y is defined as the error function L (x,y). Then, when the AF of the decoder is an identity function, L(x,y) = kx−yk 2 can be obtained.
When the AF of the decoder is Sigmoid function, Eq (11) can be obtained: When S ¼ fX ðiÞ g N i¼1 is a given training sample set, the overall loss function of AE can be expressed by Eq (12): Finally, GDM is used to calculate the minimum of iteratively J AE (θ) to determine the AE parameters. Then, the training process of AE can be ended [25,26].
(3) SAE. SAE has certain requirements for the sparsity of neuron activation degree in the hidden layer. When the input data is x, the activation degree of neuron j in the hidden layer can be expressed as h j (x), and then the Eq (13) can be obtained: In Eq (13),r j -average activation of the hidden layer neuron j on the sample set Thus, to make the sparsity of neurons in the hidden layer meet the constraints, r j ¼ r; ðj ¼ 1; 2; . . . ; mÞ, where ρ is a small sparsity parameter (ρ = 0.05). If ρ andr j are too far apart, KL(Kullback-Leibler) divergence shall be used to adjust it. The function of KLðr k r j Þ is defined as in Eq (14): The greater the difference between ρ andr j , the larger KLðr kr j Þ will be. When r ¼r j , the function value reaches the minimum [27][28][29][30]. Therefore, if KLðr kr j Þ is added to the AE loss function, then ρ andr j get as close as possible when r ¼r j . Accordingly, the loss function of SAE can be obtained, as in Eq (15): In Eq (15), β-the weight coefficient to control the sparsity penalty. After the loss function J SAE (θ) is minimized, the parameter θ can be obtained.
The previous section has expounded on the relevant theories of SAE and AE. Their difference is explained as follows. The SAE can be obtained by adding the corresponding regular restriction on the basis of AE. Comparing the two encoders reveals that the SAE can impose constraints on the loss function. SAE reflects the unique statistical characteristics of the training data set rather than simply acting as an identity function. By training the data in this way, a model can be designed to learn useful features. Finally, numerous training parameters will complicate the training process. The dimension of training output is much higher than that of input. This will produce much redundant data information. Adding SAE can add value to the learned features, which is also in line with the characteristics of the sparse response of human brain neurons.
AE can reconstruct the input data. The middle layer of an AE is the feature expression of the input data. The stacked AE method removes the output layer of the previous AE and imports the feature map of the hidden layer to the next AE. Accordingly, the Deep Neural Network (DNN) structure of the Stacked AE can be formed. A Softmax classifier is connected at the end of the Stacked AE structure for classification operation. The Stacked AE DNN is trained in layers, including the final classifier. Each layer is trained by the back-propagation algorithm. The structure of DNN constructed based on sparse edge noise reduction AE is demonstrated in Fig 3: After the DNN model of AE based on sparse edge noise reduction is constructed, the layerwise greedy training algorithm is used for training. It mainly includes two stages: pre-training and fine-tuning. The algorithm flow is listed in Table 1: https://doi.org/10.1371/journal.pone.0270308.g003 Table 1. Algorithm flow chart.

Input:
Train: The training set is used to train the network model and determine its parameters.

Valid:
The validation set is used to determine the optimal network model. 1) A two-layer stacking network model is constructed to determine the size of the block minibatch of the three data sets.
2) Through the network model constructed in (1), the calculation method of the pre-training loss function of the model is obtained.

3)
For hidden layer i the network model: For an epoch in pretrain_epoches: For a batch_index in minibatch: Call the loss function calculation method obtained in (2) to calculate the loss after each hidden layer is encoded and decoded. Use the BP algorithm to adjust the model parameters.

4)
Output the average loss of an epoch in each layer.

5)
The model parameters are determined through the network model constructed in (1). The calculation method of the fine-tuning stage of the model is obtained.

7)
Keep iterating to find the best_validation_loss of this_validation_loss in the test set. Update best_validation_loss. The core idea of the layer-by-layer greedy algorithm is to construct an optimal solution step by step. Each step makes an optimal decision under certain standards, and the decision made in each step cannot be changed in the next steps. For example, for the loading of containers, load the containers onto the arrival ship step by step, and load one container step by step. Each step determines which container to load. The greedy criterion for making decisions is to select the container with the smallest weight from the remaining containers to ensure the minimum total weight of the selected container. The cargo ship can load more containers with the maximum capacity. Additionally, the knapsack problem also involves a layer-by-layer greedy algorithm. For a knapsack with n items and a capacity of c, the packed items are selected from n items. The weight of article i is w i . The value is p i . A feasible knapsack loading means that the total weight of the packed items does not exceed the capacity of the knapsack. An optimal backpack load refers to the feasible backpack load with the highest total value of goods. The backpack problem can be described by Eq (16)- (17): The constraints are: In Eqs (16)- (17), the value of x i must be calculated. x i = 1 means that the item i is loaded into the backpack. Otherwise, x i = 0 means item i is not loaded into the backpack.

Translation language information extraction
This section sets the hyperparameter as the translation quality of unreferenced English articles. The language information is fused through the automatic translation language-oriented information extraction method. In the context of DL, the learning and training process of language information extraction method for translation of English articles without reference consists of two stages: the supervised learning stage and the unsupervised learning stage. In the unsupervised learning stage, the AE is mainly used to learn and train the words of the Source Language (SL) and Target Language (TL), and then the bilingual semantic features of the two languages are obtained. In the supervised learning stage, to optimize the extraction effect of language vector features, it is necessary to import the standard information of Natural Language (NL) corpora into bilingual words to realize the fine-tuning of bilingual semantic features [31,32].
The unsupervised learning stage takes the training corpus of SL A and TL B as the learning objects. Fig 4 demonstrates the learning diagram: The unsupervised learning stage learns the training corpus of source language A and translation results of target language B. Translation result of B must not be entirely consistent with A. Then, the automatic MT system is trained by vector A (Y E ) and vector B (Y C ) to obtain the bilingual aligned sample pairs (Y E , Y C ). (Y E , Y C ) represents a bilingual word. Based on this, noise reduction AE is used to learn the bilingual word (Y E , Y C ) unsupervised to reconstruct the new vector A and vector B for vector B in sample y are reconstructed. Finally, the language vector features in the automatic MT sample are obtained.
Next, the two NLs are learned through the AE to optimize the unsupervised learning process's reliability. Before reconstructing the vectors A, B in sample y, a certain degree of noise is introduced into the sample pair ( The AE can implicitly express k E , k C of the two NLs A, B through the Sigmoid AF, as demonstrated in Eqs (18) (19): In Eqs (18) and (19), g θ -coding function; r-AF; V E , V C -translation matrix parameters, which are bilingual words with their unique language characteristics; � Y E ; � Y C -Bilingual words; β-bias. Since k E , k C have the same dimension, β is shared by A, B.
After k E , k C is obtained, AE decodes the implicit expressions of A and B, and k E is decoded to obtain the reconstructed vectorŷ c of TL and reconstruction vector y of SLŷ E . Eqs (20) (21) specify the results:ŷ In Eqs (20) and (21), g θ -decoding function; d C , d E -decoder bias of language. The decoding method of implicit k E is the same as the decoding steps k C . Then, implicit k E can be decoded to obtainŷ E , and the implicit k C can be decoded to obtainŷ C . This kind of encoding and decoding form can reconstruct one NL to get the vector of another NL. It can also be transformed into the vector of the SL. However, due to the difference in information between the two NLs, some errors will be caused in reconstruction. Accordingly, Y E is reconstructed to obtain the SL vector and the corresponding error p(Y E ). Meanwhile, Y C is reconstructed to obtain the original vector and the corresponding error p (Y C ). Afterward, Y C is reconstructed to obtain the SL vector Y E and the corresponding error p is reconstructed to obtain the error of the original vector pair p½ðY C ; Y E Þ; ðŷ C ;ŷ E Þ�. Lastly, the reconstruction error of the sample pair (Y E , Y C ) is set as the cross-entropy, and the sum of five kinds of reconstruction errors p( is set as the loss function in the unsupervised learning stage [33][34][35].
The decoding function in the unsupervised learning process is set as g θ = {V E , V C , d E , d C }, which the GDM updates to minimize the loss function, and V E , V C is obtained after training.

TQA of unreferenced English articles
The DL-based unreferenced English articles-oriented TQA model is composed of one regression layer, one hidden layer, and three visual layers, which are represented by t 1 , t 2 , t 3 respectively. The trained V E , V C will be input to the visual layer, the three hidden layers contain 100 nodes, and the regression layer contains one node and outputs the results. The Joint Probability Distribution (JPD) of the hidden layer and visible layer is calculated by Eq (22): In Eq (22), Q(t 1 )�Q(t 2 )�Q(t 3 )-the probability that the semantic variables of hidden layers t 1 , t 2 , t 3 meet the translation needs.
The evaluation steps of the AE-based automatic TQA model for unreferenced English articles mainly include three steps. Firstly, the DL network is trained unsupervised from top to bottom. Each network layer is set as a Restricted Boltzmann Machine (RBM). The weights of each layer of the network are trained through a greedy learning algorithm, and the layered training is carried out from bottom to top. The first layer and other network layers are modeled respectively to obtain binary-binary RBM and Gaussian-Binary RBM. Hidden nodes and visible nodes are independent of each other in the RBM, and Eq (23) expresses their conditional probability distribution: Then, Eq (24) manifests the JPD: In Eq (24), M()-Gaussian density function; logistic-logical function; f j -bias of visual layer; t 1 -the number of nodes in the hidden layer is 1; ε-standard deviation; ϕ-weight.
Secondly, supervised overall fine-tuning is performed on the output layer according to the input bilingual word V E , V C .
Lastly, the bias and weight of each layer are obtained through supervised learning and unsupervised training, and a regression model that can output the characteristics of the translated article is implemented. Then, the translation quality is evaluated by using the proposed regression model. The proposed regression model is manifested in Eq (25): In Eq (25), O-automatic evaluation result of translation quality of unreferenced English articles; ϕ-weight.
Further, the translation results of different software come from the average score of 30 English professors. The calculation method is given in Eq (26).
In Eq (26), A i represents the score given by the ith professor on the current evaluation index.

Model validity
This section takes the statements in a news website as the experimental data set based on the index evaluation standard of the Translation Service Specification Part 1: Translation compiled by China Translation Association. Then, the statement translation quality is automatically evaluated through the proposed regression model.  When the effectiveness of the proposed model is evaluated, the difference between the actual situation and the automatic evaluation structure is set as the test index, and the test results of the proposed model are illustrated in Fig 7: Comparing Figs 6 and 7 find that the difference between the evaluation results of correct sentences and the number of correctly translated sentences of the proposed model is 1. This is because there are differences in syntactic structures between different languages. Therefore, the translation quality evaluation of unreferenced English articles by the proposed model can meet the actual needs and, thus, can effectively evaluate the translation quality.

Influence of sentence numbers on the proposed TQA model's performance
The actual translation quality and the evaluation result of the proposed TQA model for a given sentence are denoted as A1 and A2, respectively. The ratio between A2 and the correct As plotted in Fig 9, when the number of sentences increases, the number of actual translation errors and the evaluation scores of the proposed TQA model increase, but the BLEU score is not significantly affected. When the number of sentences increases from 1,000 to 6,000, the BLEU score increases from 96 to 98, which shows that the proposed model has good performance. From the discourse perspective, Fig 10B mainly analyzes the software translation's cohesion, coherence, intention, acceptability, information, context, and intertextuality. Apparently, software C has fewer errors in vocabulary and grammar than software A and B. However, its error rate in text translation is higher.
Next, the proposed model is compared with other literature methods: "Quality Evaluation of Machine Translation of Chinese Passive Voice-Taking Google translation and Youdao translation as an example" and "Research on Machine Translation Quality Evaluation Method Based on Questionnaire and Data Analysis." Fig 11 compares the evaluation accuracy of different methods to evaluate the proposed model's effectiveness: Fig 11 indicates that when the number of sentences is 1,000-6,000, compared with the methods in literature 1 and 2, the proposed model is obviously better than the other two models. The proposed model's prediction accuracy is as high as 97%, and the stability is better, so it has a certain application value. Further, to further evaluate the application value of the proposed model, Fig 12 compares the quality evaluation efficiency of the three models: As described in Fig 12, given 1,000-6,000 statements, the efficiency of the proposed TQA model for unreferenced English articles is significantly better than the other two literature methods, with the highest quality evaluation efficiency reaching over 95%. The accuracy of the translation quality evaluation can be guaranteed to a certain extent, indicating that the proposed model is highly feasible.

Comparative analysis of algorithms
So far, BLEU is a widely used TQA method, whether it be the traditional MT statistic analysis or the popular NN-based MT evaluation or training. Although many automatic TQA methods have received higher manual evaluation correlation than BLEU, BLEU remains the most widespread TQA. Generally, MT systems are trained by the Minimum Error Rate Training (MERT) based on multi-feature representation. MERT optimizes the MT system by minimizing the translation error rate. Specifically, the feature weight is optimized on the development set to gain the optimal automatic evaluation index. Common evaluation indexes include BLEU, Testing Error Rate (TER), and METEOR. In order to control the possible impact of each system module, the same MT system is trained and used in the subsequent experiment. The MT system chooses a Translation Model (TM) trained from the Linguistic Data Consortium (LDC)-provided bilingual corpus. This corpus included LDC2002E18, LDC2003E14, LDC2004E12, LDC2004T08, LDC2005T10, and LDC2007T09 data sets, totaling 8.3 million pairs of Chinese-English translation data. The Chinese part uses the ICT-CLAS word segmentation tool. The language model (LM) is trained by a monolingual corpus composed of the After being trained by BLEU, TER, and METEOR, the TER-trained MT system has the translation missing issue, and the translated sentences are too short. Simultaneously, the BLEU-trained system also has a small amount of missing translation but shows a muchimproved translation quality than the TER-trained system. By comparison, the METEOR system over translates and generates long redundant sentences. Overall, the BLEU-trained MT system presented a stable performance in all evaluation indexes. The findings reveal why the BLEU evaluation indexes are chosen in the present work.

Conclusions
MT evaluation is divided into two directions: one evaluates the MTS referring to prior human translations, and the other evaluates the MTS without reference to artificial translation. Most of the work focuses on the first task. In the past decade, researchers have done a lot of work on the second task. Indisputably, a suitable evaluation method helps improve the performance of the MTS. Therefore, the rapid development of MT drives the continuous development of evaluation technology. At the same time, more and more evaluation activities also promote the constant development of translation evaluation technology. In return, competitive evaluation activities promote MT performance to a certain extent.
Thereupon, this paper proposes an unreferenced English articles-oriented automatic TQA model based on SAE under the background of DL. Then, the DL-based automatic translation language-oriented information extraction method employs AE in the bilingual words' unsupervised learning stage. It reconstructs the language vector features of the sample unreferenced English articles to be translated. Afterward, it imports the translation information of unreferenced English articles into bilingual words and optimizes the extraction effect of language vector features. The translation language vector feature is introduced into the proposed DL-based automatic TQA model. The experimental results corroborate that the difference between the evaluation results of correct sentences and the number of correctly translated sentences is 1, and the accuracy of the evaluation results is high. When the number of sentences increases, the number of actual translation errors and the evaluation scores of the model increase, but the BLEU is not significantly affected. When the number of sentences increases from 1,000 to 6,000, the BLEU increases from 96 to 98, which shows that the proposed TQA model has good performance.
However, there are still some problems with the research content. Due to the complexity of translation activities, there are still some difficulties in the comprehensive evaluation of translation. At present, there is no definite conclusion on the evaluation criteria of translation, and there are differences in evaluation parameters and weights, so it isn't easy to propose an evaluation model that meets all translation requirements and has strong operability. Therefore, to promote the development of automatic evaluation technology, future work will focus on the above problems.